The present invention relates to a masking device for a flat-screen colour-display cathode-ray tube, of the type comprising a support frame for a tensioned shadow mask and to a tensioned shadow mask mounted on the support frame.
Colour-display cathode-ray tubes comprise, in a known manner, a display screen provided with phosphors, an electron gun producing three electron beams and a masking device consisting of a shadow mask mounted on a support frame, placed opposite the display screen and intended to ensure that the image displayed is of high quality. The shadow mask consists of a metal foil perforated by a plurality of holes or slits through which the three electron beams pass in order to excite the phosphors disposed on the screen. The image quality obtained is better the more precise the alignment between the phosphors, the holes in the shadow mask and the electron beams. When the display tube is in operation, a significant portion of the electron beams is intercepted by the shadow mask. This causes local heating of the shadow mask, which may deform it, and therefore the quality of the displayed image may suffer. In addition, the image quality may also suffer owing to the vibrations of the shadow mask which are caused by various vibration sources. In order to obtain high-quality images, the shadow mask must, on the one hand, be insensitive to the local heating and, on the other hand, it must have a natural vibration frequency high enough for the amplitude of these vibrations not to disturb the colour of the images by a misalignment between the electron beams, the holes in the shadow mask and the phosphors.
When the display screen is curved, the shadow mask has a shape which matches that of the screen, and the problems of sensitivity to local heating and of vibration are solved by making the shadow mask by drawing a foil, made of an Fexe2x80x94Ni alloy having a very low thermal expansion coefficient, perforated by holes. The shadow mask is simply welded onto a support frame which exerts no force on the shadow mask. The frame may therefore be lightweight, which has advantages.
When the display screen is flat, the shadow mask may be an undrawn foil fixed, for example, by welding to a precompressed support frame which then exerts tension on the shadow mask. The shadow mask is then referred to as a xe2x80x9ctensionedxe2x80x9d mask. The tension in the shadow mask is intended, on the one hand, to solve the problem of sensitivity to local heating and, on the other hand, to increase the natural vibration frequency of the shadow mask in order to attenuate the amplitude of these vibrations. This solution assumes, in particular, the use of a material whose characteristics allow sufficient tension to be maintained in the operating temperature range of the cathode-ray tube (approximately 100xc2x0 C.), and to do so after heating to about 600xc2x0 C. during the process for manufacturing the cathode-ray tube. This is because the shadow mask, mounted so as to be tensioned on its support frame, is heated a first time to around 600xc2x0 C., in order to cause what is called xe2x80x9cblackeningxe2x80x9d oxidation, and then a second time to around 450xc2x0 C. after the assembly has been mounted in the cathode-ray tube, during the operation of sealing the screen-tile to the glass cone, and finally a third time in the region of 380xc2x0 C. when evacuating the cathode-ray tube. These heating operations may cause the shadow mask and its frame to creep, which may relax the shadow mask.
In order to manufacture a tensioned shadow mask and its support frame, it has been proposed to use a low-alloy steel (that is to say one containing, in general, less than 5% of alloying elements). However, since the thermal expansion coefficient of this steel is high, the tension in the shadow mask must be greater than 200 MPa in order to prevent deformations due to local heating. This solution results in a heavy frame, the weight of which may reach, or even exceed, 6 kg.
In order to manufacture a tensioned shadow mask and its support frame, it has also been proposed to produce the shadow mask from an Fexe2x80x94Ni alloy having a low expansion coefficient and the frame from steel. However, it is then necessary to provide means for preventing the shadow mask from being over-tensioned during heating at 600xc2x0 C., without which the shadow mask would tear during this operation.
To manufacture a tensioned shadow mask and its support frame, it has also been proposed to produce the shadow mask and the support frame from Fexe2x80x94Ni alloys having a low expansion coefficient, it being possible for the Fexe2x80x94Ni alloy of the support frame to be identical to or different from the Fexe2x80x94Ni alloy of the shadow mask. This solution may cause defects within the shadow mask, these defects being visible after heating to 600xc2x0 C. This is because the support frame, of rectangular overall shape, has two end uprights to which the shadow mask is attached and two lateral uprights which ensure that the separation between the end uprights is maintained. The shadow mask, also of rectangular overall shape, is attached to the end uprights, generally by welding, along two of its opposed sides. In any event, the tension exerted on the shadow mask in the longitudinal direction generates a tension in the transverse direction. During heating to high temperature, these tensions may cause creep phenomena which, because of the holes or slits in the shadow mask, may cause an elongation in the transverse direction of the shadow mask. If during heating to 600xc2x0 C. the end uprights of the support frame expand as much as or more than the shadow mask, the initial tension in the transverse direction will be retained or increased. After returning to room temperature, the end uprights of the support frame resume their original dimensions, whereas the shadow mask has a slightly increased width because of the creep. This phenomenon may result in corrugations in the shadow mask, making it unusable. This defect, which is more pronounced the larger the shadow mask, may be aggravated by the fact that, on cooling after the 600xc2x0 C. hold, the shadow mask cools more quickly than the frame.
It is an object of the present invention to remedy these drawbacks by providing a means for manufacturing a tensioned shadow mask and its support frame which are insensitive to the local heating, having a suitable natural vibration frequency and having good flatness after the high-temperature heating resulting from the manufacturing operations.
For this purpose, the subject of the invention is a masking device for a flat-screen colour-display cathode-ray tube, of the type comprising a support frame for a tensioned shadow mask and a tensioned shadow mask mounted on the support frame so as to be subjected to tension at room temperature. The support frame is made of a hardened Fexe2x80x94Ni alloy having a thermal expansion coefficient between 20xc2x0 C. and 150xc2x0 C. of less than 5xc3x9710xe2x88x926 Kxe2x88x921 and a yield stress Rp0.2 at 20xc2x0 C. of greater than 700 MPa. The tensioned shadow mask is made of an Fexe2x80x94Ni alloy having a thermal expansion coefficient between 20xc2x0 C. and 150xc2x0 C. of less than 3xc3x9710xe2x88x926 Kxe2x88x921. The hardened Fexe2x80x94Ni alloy of which the support frame is made and the Fexe2x80x94Ni alloy of which the shadow mask is made are chosen in such a way that, below a temperature T1, the mean expansion coefficient xcex120-T, between 20xc2x0 C. and the temperature T, of the hardened Fexe2x80x94Ni alloy of which the support frame is made is greater than the mean expansion coefficient xcex120-T, between 20xc2x0 C. and the temperature T, of the Fexe2x80x94Ni alloy of which the shadow mask is made; above the said temperature T1, the mean expansion coefficient xcex120-T, between 20xc2x0 C. and the temperature T, of the hardened Fexe2x80x94Ni alloy of which the support frame is made is less than the mean expansion coefficient xcex120-T, between 20xc2x0 C. and the temperature T, of the Fexe2x80x94Ni alloy of which the shadow mask is made; the said temperature T1 is less than 350xc2x0 C. and preferably less than 300xc2x0 C.
Preferably, the hardened Fexe2x80x94Ni alloy of which the support frame is made is an Fexe2x80x94Ni alloy of the xe2x80x9cxcex3xe2x80x2-hardenedxe2x80x9d type whose chemical composition comprises, by weight:
40.5%xe2x89xa6Ni+Co+Cuxe2x89xa643.5%
0%xe2x89xa6Coxe2x89xa65%
0%xe2x89xa6Cuxe2x89xa63%
1.5%xe2x89xa6Tixe2x89xa63.5%
0.05%xe2x89xa6Alxe2x89xa61%
Cxe2x89xa60.05%
Sixe2x89xa60.5%
Mnxe2x89xa60.5%
Sxe2x89xa60.01%
Pxe2x89xa60.02%
the balance being iron and impurities resulting from the smelting,
and the Fexe2x80x94Ni alloy of which the shadow mask is made is an Fexe2x80x94Ni alloy whose composition comprises, by weight:
32%xe2x89xa6Ni+Co+Cuxe2x89xa637%
0%xe2x89xa6Coxe2x89xa65.5%
0%xe2x89xa6Cuxe2x89xa62%
0%xe2x89xa6Nb+Ta+Mo+W+Zrxe2x89xa62%
0xe2x89xa6Mnxe2x89xa60.5%
Si less than 0.2%
C less than 0.02%
S less than 0.01%
P less than 0.02%
the balance being iron and impurities resulting from the smelting.
The chemical composition of the Fexe2x80x94Ni alloy of which the shadow mask is made may, for example, be such that:
32%xe2x89xa6Ni+Co+Cuxe2x89xa635.5%
0%xe2x89xa6Coxe2x89xa64%
0%xe2x89xa6Cuxe2x89xa62%
0%xe2x89xa6Nb+Ta+Mo+W+Zr less than 0.2%.
The chemical composition of the Fexe2x80x94Ni alloy of which the shadow mask is made may likewise be such that:
33.5%xe2x89xa6Ni+Co+Cuxe2x89xa637%
0%xe2x89xa6Coxe2x89xa65.5%
0%xe2x89xa6Cuxe2x89xa62%
0.2%xe2x89xa6Nb+Ta+Mo+W+Zrxe2x89xa62%.
In another embodiment, the hardened Fexe2x80x94Ni alloy of which the shadow frame is made may be an Fexe2x80x94Ni alloy of the xe2x80x9cxcex3xe2x80x2-hardenedxe2x80x9d type whose chemical composition comprises, by weight:
43.5%xe2x89xa6Ni+Co+Cuxe2x89xa645.5%
0%xe2x89xa6Coxe2x89xa65%
0%xe2x89xa6Cuxe2x89xa63%
1.5%xe2x89xa6Tixe2x89xa63.5%
0.05%xe2x89xa6Alxe2x89xa61%
Cxe2x89xa60.05%
Sixe2x89xa60.5%
Mnxe2x89xa60.5%
Sxe2x89xa60.01%
Pxe2x89xa60.02%
the balance being iron and impurities resulting from the smelting,
and the Fexe2x80x94Ni alloy of which the shadow mask is made may be an Fexe2x80x94Ni alloy whose chemical composition comprises, by weight:
35.5%xe2x89xa6Ni+Co+Cuxe2x89xa637%
0%xe2x89xa6Coxe2x89xa65.5%
0%xe2x89xa6Cuxe2x89xa62%
0xe2x89xa6Mnxe2x89xa60.5%
Si less than 0.2%
C less than 0.02%
S less than 0.01%
P less than 0.02%
the balance being iron and impurities resulting from the smelting.
The invention also relates to a tensioned shadow mask consisting of an Fexe2x80x94Ni alloy whose chemical composition comprises, by weight:
32%xe2x89xa6Ni+Co+Cuxe2x89xa637%
0%xe2x89xa6Coxe2x89xa65.5%
0%xe2x89xa6Cuxe2x89xa62%
0%xe2x89xa6Nb+Ta+Mo+W+Zrxe2x89xa62%
0xe2x89xa6Mnxe2x89xa60.5%
Si less than 0.2%
C less than 0.02%
S less than 0.01%
P less than 0.02%
the balance being iron and impurities resulting from the smelting.
The chemical composition of the Fexe2x80x94Ni alloy of which the shadow mask is made may preferably be such that:
32%xe2x89xa6Ni+Co+Cuxe2x89xa635.5%
0%xe2x89xa6Coxe2x89xa64%
0%xe2x89xa6Cuxe2x89xa62%
0%xe2x89xa6Nb+Ta+Mo+W+Zrxe2x89xa60.2%.
The chemical composition of the Fexe2x80x94Ni alloy of which the shadow mask is made may likewise be such that:
33.5%xe2x89xa6Ni+Co+Cuxe2x89xa637%
0%xe2x89xa6Coxe2x89xa65.5%
0%xe2x89xa6Cuxe2x89xa62%
0.2%xe2x89xa6Nb+Ta+Mo+W+Zrxe2x89xa62%.
Finally, the frame may also be made of a hardened Fexe2x80x94Ni alloy of the xe2x80x9cberyllium-hardenedxe2x80x9d type, of the xe2x80x9ccarbide-hardenedxe2x80x9d type or of the xe2x80x9csolid-solution-hardenedxe2x80x9d type.